Nev mesh being generating weird zone

Hi, so as the title implied why does my nav mesh generate such a weird map? Attached picture 1 is without any changes made to setting on RecastNavMesh. Picture 2 when i changed the agent radious to 0. The furniture and house itself i made it and import as FBX from blender. Is the collision the root of my problem? i tried adding new second nav mesh, but it seemed like the system wont allow. Also, I would also like the stairs to be connected on the second floor, any idea how to do it?

thank you so much.